The altitude correction box is a simple on/off switch. The altitude correction box output  is an open circuit at ground level. It closes (short circuit) above 3,280 ft. (1000m). It connects to pin 30 of the DME connector. The DME responds to the signal of the altitude correction box by leaning out the fuel to 94%. 

The altitude correction box , Porsche part no.944 606 051 00, is the M157 option of the Porsche 944. Not used in the Euro/ROW versions of the 944 until 1987.